Merged variable gain mixers

ABSTRACT

The present invention teaches parallel coupling what are herein termed a “switching stage” and a “steering stage,” thereby arranging the mixer and variable gain amplifier circuitry as a single merged circuit. The merged variable gain mixers of the present invention provide mixing and gain functionality utilizing only that power needed for a basic mixer function and only the transconductance of the basic mixer function (thereby eliminating non-linearities introduced by additional transconductance stages of prior art circuitry). Further, in the merged variable gain mixers described herein, no additional headroom is needed other than what is required by the basic mixer function. The present invention contemplates a variety of merged variable gain mixers including AC and DC coupled merged variable gain mixer of both single and double balanced configuration.

DESCRIPTION

1. Technical Field

The present invention is related to signal mixing and amplifying orattenuating circuitry. More specifically, the present invention teachesa variety of merged variable gain mixers capable of mixing and variablyamplifying or attenuating an input signal.

2. Background Art

Many electronics applications require processing an input frequencysignal with both a mixing function and a variable gain function. Forexample, typical wireless communication system transmitters andreceivers utilize separate mixer and variable gain amplifier devices inorder to operate upon the input signal as desired. In many applications,such circuitry must operate with low supply voltages (e.g., 2.7 Volts)and at extremely low temperatures (e.g., −40 degrees C.).

FIG. 1 illustrates a prior art mixing and gain control circuitry 100suitable for mixing and variably amplifying an input signal VIN togenerate a desired output signal Vout. The circuitry 100 includes a socalled Gilbert cell mixer 102, a variable gain amplifier 104, blockingcapacitors C₁ and C₂, and a bias source formed by the resistor pair R₁and R₂. The Gilbert cell mixer 102 includes three differentiallyconnected transistor pairs Q1-Q2, Q3-Q4, and Q5-Q6, a current source I1,and a pair of resistors R₃ and R₄. Those skilled in the art will befamiliar with the operation of the Gilbert cell mixer 102, thefunctionality of which is defined primarily by the linear transconductor103 (formed by the transistor pair Q5-Q6) and the current switch orswitching quad 105 (formed by the transistor quad Q1-Q4) which causesthe mixing of the input current.

The input signal Vin is coupled to the bases of transistors Q5 and Q6 sothat the linear transconductor 103 may convert it into a current signal.The local oscillator signal LO is connected to the bases of differentialtransistor pairs Q1-Q2 and Q3-Q4. The blocking capacitors C₁ and C₂ mayserve to disassociate any DC bias present on the LO signal.

The mixer 102 operates as follows. The current signal out of thetransconductor 103 is multiplied by +1 and −1 at the frequency of thelocal oscillator signal LO. This happens as the local oscillator signalLO switches the switching quad 105 ON and OFF at the frequency of thelocal oscillator signal LO. The resulting output current into the load(the load here being represented for convenience by R₃ and R₄) is acurrent which contains signals with frequencies that are the sum anddifference, respectively, of the frequencies of the local oscillatorsignal LO and the input signal Vin. Typically only one of the sum anddifference signals is later used, the other being eliminated by simplefiltering (not shown in FIG. 1). Hence, as will be appreciated, themixer 102 is basically a multiplier that takes a local oscillator signalLO and the input signal Vin and frequency translates the input signalVin to a new differential signal at the mixer output terminals M₁ andM₂.

Depending upon the application, a variable gain function can eitherprecede or follow the mixer function in a radio transceiver. In atransmit application the mixer would serve as an upconvertor. That is,the input would be either at baseband or intermediate frequency (IF),typically 0-20 MHz and 10-400 MHz, respectively. The output signal fromthe mixer would be at much higher RF frequencies. Typically, anyvariable gain function preceding the mixer is called IF variable gainand any variable gain function after the mixer is called RF variablegain.

In the prior art Gilbert cell mixers there are implementations of IFvariable gain wherein the gain of the input transconductor can be variedby controlling the gain of the current source I1. However, in FIG. 1 weillustrate an RF variable gain control function (i.e., the gain functionis implemented after the mixer). Accordingly, the differential signalgenerated by the mixer at terminals M₁ and M₂ is coupled to the input ofthe variable gain amplifier 104. As will be appreciated, the amplifier104 shown in FIG. 1 is simply a conceptual representation of a voltagecontrolled variable gain amplifier. That is, as the control voltageV_(gc) varies, the gain of the amplifier 104 varies.

Prior art FIG. 2 is a schematic showing in more detail one typicalvariable gain amplifier 104. The amplifier 104 of FIG. 2 includes threedifferentially connected transistor pairs Q7-Q8, Q9-Q10, and Q11-Q12, apair of resistors R₅ and R₆, and a second current source 12. The mixeroutputs M₁ and M₂ are coupled to the bases of transistors Q7 and Q8. Theemitters of Q7 and Q8 are coupled at a first terminal of a secondcurrent source 12. Hence the voltage signal on M₁, and M₂ is once againconverted into a current signal by another transconductance stage formedby the transistor pair Q7-Q8. Those of skill in the art will appreciatethat a linear transconductance stage such as this may take oninnumerable forms.

The control voltage V_(gc) is coupled to the bases of transistor pairsQ9-Q10 and Q11-Q12. The emitters of Q9 and Q10 are coupled to thecollector of transistor Q7. The emitters of Q11 and Q12 are coupled tothe collector of transistor Q8. The collectors of transistors Q9 andQ12, and first terminals of the resistors R₅ and R₆ are coupled to thesupply voltage V_(cc). A second terminal of the resistor R₅ is coupledto the collector of transistor Q10 and a second terminal of the resistorR₆ is coupled to the collector of transistor Q11. The output signalO_(f) is therefore generated at the collectors of Q10 and Q11.

Typical of the Prior Art, the mixing and gain circuitry 100 shown inFIGS. 1 and 2 has several shortcomings. One major shortcoming is thenon-linearity introduced by the two transconductance stages (i.e., Q5-Q6and Q7-Q8). Another serious shortcoming is the power loss due to thecurrent sources I1 and I2. This power loss is particularly problematicwhen designing for supplies at 2.7V (and lower) and cold temperatureoperating conditions.

FIG. 3 illustrates a second mixing and gain control circuitry 200 of thePrior Art. The circuitry 200 is motivated by a recognition that thesecond transconductance stage formed by the transistor pair Q7 and Q8 isunnecessary if one connects the Gilbert cell mixer 102 directly inseries with the transistor pairs Q9-Q10 and Q11-Q12. Doing so, as shownin FIG. 3, allows the gain control voltage signal V_(gc) to directlycontrol the current flow through the Gilbert cell mixer 102. Thiseliminates any non-linearities introduced by the second transconductancestage as well as improving power efficiency by eliminating the secondcurrent source I2.

The improvements in linearity and power efficiency of circuitry 200 arenot free. Connecting the Gilbert cell mixer 102 in series with thevariable gain amplifier of FIG. 3 forms a circuit that suffers from afour transistor voltage drop, specifically, the three transistor dropacross the Gilbert cell mixer 102 and the single transistor drop acrosstransistor pairs Q9-Q10 and Q11-Q12. This makes use of the circuitry 200with low voltage supplies problematic.

What is needed is a mixer and variable gain amplifier circuit that lacksthe non-linearity and power inefficiencies of multiple transconductancestages, yet is capable of operating properly when provided low supplyvoltages.

DISCLOSURE OF THE INVENTION

The present invention teaches parallel coupling what are herein termed a“switching stage” and a “steering stage,” thereby arranging the mixerand variable gain amplifier circuitry as a single merged circuit. Themerged variable gain mixers of the present invention provide mixing andgain functionality utilizing only that power needed for a basic mixerfunction and only the transconductance of the basic mixer function(thereby eliminating non-linearities introduced by additionaltransconductance stages of prior art circuitry). Further, in the mergedvariable gain mixers described herein, no additional headroom is neededother than what is required by the basic mixer function. The presentinvention contemplates a variety of merged variable gain mixersincluding AC and DC coupled merged variable gain mixer of both singleand double balanced configuration.

For example, one embodiment of the present invention teaches a mergedvariable gain mixer having a switching device (or stage) and a currentsteering device (or stage) coupled in parallel. The switching device isresponsive to an oscillator signal LO applied to a first input pair andan input signal I_(f) applied to a second input pair to generate anoutput signal O_(f) at an output pair. The output signal O_(f) will havean amplitude that is a function of the input signal I_(f) and an outputfrequency that is a function of both the signal LO and the input signalI_(f). The current steering device is operable to control, as a functionof a control voltage V_(gc) applied at a control voltage input, acurrent flowing between a supply voltage V_(cc) coupled to a supplyvoltage input and a current steering device first input pair. Theswitching device second input pair and the current steering device firstinput pair are coupled together such that the current steering devicecan attenuate the output signal O_(f) by steering a portion of thecurrent of the input signal I_(f) away from the switching device anddumping that portion of the current out to the supply voltage V_(cc).

The different stages may take on a variety of different arrangements.For example, in a double balanced merged variable gain mixer, the mixingstage may be formed as a mixing quad having four transistors and thesteering stage may be formed as a steering quad with another fourtransistors. Alternatively, in a single balanced merged variable gainmixer, the mixing stage may be formed by a pair of transistors and thesteering stage may be formed by another pair of transistors.

The present invention also teaches a method for mixing and variablyamplifying an input signal I_(f) in order to generate an output signalO_(f). This method involves receiving an input signal I_(f) having aninput frequency and an input current, receiving a local oscillatorsignal, receiving a gain control voltage V_(gc), absorbing a portion ofthe input current as a function of the gain control voltage V_(gc) andthe local oscillator signal, thereby generating an attenuated inputsignal having an attenuated input current, and generating an outputsignal O_(f) that is a function of the attenuated input signal, theinput frequency, and the local oscillator signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Prior Art FIG. 1 is a schematic of one traditional mixer and variablegain amplifier circuitry.

Prior Art FIG. 2 is a schematic showing the variable gain amplifier ofFIG. 1 in more detail.

Prior Art FIG. 3 is a schematic of another traditional coupling of amixer and a variable gain amplifier.

FIG. 4 is a schematic of an AC coupled, double balanced, merged variablegain mixer in accordance with one embodiment of the present invention.

FIG. 5 is a schematic of a DC coupled, double balanced, merged variablegain mixer in accordance with another embodiment of the presentinvention.

FIG. 6 is a schematic of an AC coupled, single balanced, merged variablegain mixer in accordance with yet another embodiment of the presentinvention.

FIG. 7 is a schematic of a DC coupled, single balanced, merged variablegain mixer in accordance with a separate embodiment of the presentinvention.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention teaches parallel coupling what are herein termed a“switching stage” and a “current steering stage,” thereby arranging themixer and variable gain amplifier circuitry as a single merged circuit.The merged variable gain mixers of the present invention provide mixingand gain functionality utilizing only that power needed for a basicmixer function and only the transconductance of the basic mixer function(thereby eliminating non-linearities introduced by additionaltransconductance stages of prior art circuitry). Further, in the mergedvariable gain mixers described herein, no additional headroom is neededother than what is required by the basic mixer function. In essence, thepresent invention provides the variable gain function almost free:“almost” because due to a larger power required of the local oscillatorsignal LO, there is a slight increase in power consumption by the LOcircuitry as compared to the basic mixer function taken alone.

FIG. 4 illustrates an AC coupled, double balanced, merged variable gainmixer 300 in accordance with one embodiment of the present invention.The merged variable gain mixer 300 of FIG. 4 includes a switching quadhaving four transistors Q1, Q2, Q3, and Q4, and a steering quad havingfour transistors Q5, Q6, Q7, and Q8. The bases of transistors Q1 and Q2are coupled together and then connected to a first input LO1 for thelocal oscillator signal through a DC blocking capacitor C₁. The bases oftransistors Q3 and Q4 are coupled together and then connected to asecond input LO2 for the local oscillator signal through a DC blockingcapacitor C₂. The emitters of transistors Q1, Q3, Q5, and Q6 are coupledtogether forming a first input IF1 for the differential input signalcurrent I_(f). The emitters of transistors Q2, Q4, 7, and Q8 are coupledtogether forming a second input for the input signal I_(f).

The bases of transistors Q5 and Q8 are coupled together and thenconnected both to the first oscillator input LO1 through a DC blockingcapacitor C₃, and to a control voltage V_(b)+V_(gc) through resistors R₁and R₄. The bases of transistors Q6 and Q7 are coupled together and thenconnected both to the second oscillator input LO2 through a DC blockingcapacitor C₄, and to the control voltage V_(b)+V_(gc) through resistorsR₂ and R₃. As will be appreciated, LO1 and LO2 constitute thedifferential parts of the local oscillator signal LO. Bias current isprovided to the bases of transistors Q1 and Q2 and the bases oftransistors Q3 and Q4 through resistors R₅ and R₆, respectively.

The merged variable gain mixer 300 generally operates as follows. Toachieve the maximum gain (i.e., minimum attenuation), the controlvoltage V_(gc) is adjusted such that the steering quad transistors Q5-Q8are turned off. When completely turned off, no current is dumped throughthese transistors, instead the entire current is steered into the outputsignal O_(f). Note that in this state the merged variable gain mixer 300operates as a regular mixer.

To attenuate the output signal O_(f), the control voltage V_(gc) isadjusted to create controlled offset between transistor pairs Q1-Q5,Q3-Q6, Q4-Q7, and Q2-Q8. The combination of DC offset and the ACcoupling of the local oscillator signal LO causes the controlled offsetto be dynamic. That is, the offset is maintained over the duration ofthe entire LO cycle. This dynamic effect causes a portion of the inputcurrent, during switching, to be diverted or steered away from theoutput switched current. This steered current is essentially thrownaway, directly to a current supply for example. The amount of currentsteered away is dependent on the controlled offset.

In a typical radio transceiver, it is usually desirable to obtain alog-linear relationship between the output current and the controlvoltage. Those of skill in the art will be well familiar with suchcontrol circuits necessary to effect this relationship, and willrecognize that the merged variable gain mixers of the present inventionwork well with such prior art control circuits.

In the merged variable gain mixer 300, the blocking capacitors C₁-C₄serve to disassociate the switching quad from any DC present in thelocal oscillator circuitry. However, the local oscillator circuitrycould also be designed to include a bias signal for driving theswitching quad, thus eliminating the need to provide a bias signalthrough resistors R₅ and R₆. In this case, the blocking capacitors C₁-C₄must be removed and the control offset voltage between the two quadsintroduced through separate local oscillator signals LO. One suchsuitable embodiment will now be described.

FIG. 5 illustrates a DC coupled, double balanced, merged variable gainmixer 400 in accordance with another embodiment of the presentinvention. The merged variable gain mixer 400 is designed for use withlocal oscillator circuitry that is DC coupled. Accordingly, no blockingcapacitors are required in the variable gain mixer 400 of FIG. 5. In allother respects, the merged variable gain mixer 400 is arranged andoperates as the merged variable gain mixer 300 of FIG. 4.

The two LO waveforms LO1 and LO2 are substantially identical as far asAC characteristics are concerned. The average values are different,however, due to the offset introduced via control circuitry. Thisdynamic offset acts in exactly the same manner as described previouslyin steering current away during switching. The relationship betweenoutput current and input current follows the well known equation:$\frac{Iout}{Iin} = {A \times \frac{1}{1 + {\exp ( \frac{Vcontrol}{Vt} )}}}$

where A is a constant dependent upon the conversion gain of the mixerdue to the LO switching, Vcontrol is the offset voltage between the mainswitching devices and the auxiliary steering devices and Vt is thethermal voltage given by the well-known relationship${Vt} = {\frac{kT}{\sigma}.}$

When Vcontrol>4×Vt (about 100 mV @25 C), all current is carried by theswitching devices and the steering devices are shut off at all pointsduring the LO cycle. This is the point of maximum gain. At Vcontrol=Vt,the gain reduces to about one-half (½) the value of the maximum gain.Below Vt, the gain decreases substantially exponentially.

To further emphasize certain aspects of the present invention, FIG. 5shows the merged variable gain mixer 400 with the switching quad drawnseparately as a switching device 402 and the steering quad drawnseparately as a current steering device 404. As will be apparent, theswitching device 402 and the current steering device 404 are coupled inparallel. Hence the merged variable gain mixers of the present inventionhave no additional constraints in terms of extra supply voltage orcurrent needed as compared to a stand alone mixer of the same or similarconfiguration (e.g., the switching transistors). Those skilled in theart will appreciate that the principles of the present invention can beused to couple any suitable current steering device together with anysuitable switching device to form a merged variable gain mixer with theabove-described improved characteristics.

The above description focused upon the merged variable gain mixer in adouble balanced configuration. However, the present invention furthercontemplates single balanced merged variable gain mixers, both AC and DCcoupled. FIG. 6 provides a schematic of an AC coupled, single balancedvariable gain mixer 500, while FIG. 7 provides a schematic of a DCcoupled, single balanced variable gain mixer 600.

Like the above described merged variable gain mixers, each variable gainmixer 500 and 600 has a switching stage coupled in parallel with asteering stage. However, in order to implement a single balanced mergedvariable gain mixer, note that only four transistors (as opposed toeight) are required. By referring to FIGS. 6 and 7, and by way ofanalogy with the above-description of the double balanced variable gainmixers, those of skill in the art will readily understand the operationand implementation of the single balanced variable gain mixers of thepresent invention.

The merged variable gain mixers of the present invention do place agreater power demand upon the local oscillator circuitry. For example,in Prior Art FIG. 1 the local oscillator circuitry must drive only four(4) transistors, whereas in the embodiments of FIGS. 4 and 5 the localoscillator circuitry must drive eight (8) transistors. This is notbelieved to be a significant constraint.

Although only a few embodiments of the present invention have beendescribed in detail herein, it should be understood that the presentinvention may be embodied in many other specific forms without departingfrom the spirit or scope of the invention.

For example, the symbol utilized to denote transistors in the Figures isgenerally known to represent bipolar type transistor technology.However, it will be appreciated that field-effect transistors (FETs)such as MOSFETs would work well for the present invention.

As will further be appreciated, the present invention is suitable foruse in implementing both up and down converting mixers.

Therefore, the present examples and embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein, but may be modified within the scope of theappended claims.

I claim:
 1. A merged variable gain mixer comprising: a switching devicehaving first and second input pairs and an output pair, the switchingdevice responsive to a signal LO applied to the first input pair and aninput signal I_(f) applied to the second input pair to generate anoutput signal O_(f) at the output pair, the output signal O_(f) havingan amplitude that is a function of the input signal I_(f) and an outputfrequency that is a function of both the signal LO and the input signalI_(f); and a current steering device having a first input pair, a supplyvoltage input, and a control voltage input, the current steering deviceoperable to control, as a function of a control voltage V_(gc) appliedat the control voltage input, current flowing between a supply voltageV_(cc) coupled to the supply voltage input and the current steeringdevice first input pair, wherein the switching device second input pairand the current steering device first input pair are coupled togethersuch that the current steering device can attenuate the output signalO_(f) by steering a portion of the current of the input signal I_(f)away from the switching device and dumping that portion of the currentout to the supply voltage V_(cc).
 2. A merged variable gain mixer asrecited in claim 1, wherein the switching device includes fourtransistors Q1, Q2, Q3, and Q4, the bases of transistors Q1 and Q2coupled to a first terminal of the switching device first input pair,the bases of transistors Q3 and Q4 coupled to a second terminal of theswitching device first input pair, the emitters of transistors Q1 and Q3coupled to a first terminal of the switching device second input pair,the emitters of transistors Q2 and Q4 coupled to a second terminal ofthe switching device second pair, the collectors of transistors Q1 andQ4 coupled to a first terminal of the switching device output pair, andthe collectors of transistors Q2 and Q3 coupled to a second terminal ofthe switching device output pair.
 3. A merged variable gain mixer asrecited in claim 2 wherein the four transistors Q1, Q2, Q3, and Q4 arebipolar transistors.
 4. A merged variable gain mixer as recited in claim2 wherein the four transistors Q1, Q2, Q3, and Q4 are field-effecttransistors.
 5. A merged variable gain mixer as recited in claim 2,wherein the current steering device further includes a second input paircoupled to the switching device first input pair.
 6. A merged variablegain mixer as recited in claim 2 wherein the current steering deviceincludes four transistors Q5, Q6, Q7, and Q8, the bases of transistorsQ5 and Q8 being coupled to the first terminal of the switching devicefirst input pair, the bases of transistors Q5 and Q8 further coupled tothe control voltage input through a resistor R₁, the base of Q6 coupledto the control voltage input through a resistor R₂, the base of Q7coupled to the control voltage input through a resistor R₃, thecollectors of transistors Q5, Q6, Q7, and Q8 coupled together at thesupply voltage input, the emitters of transistors Q5 and Q6 coupledtogether at the first terminal of the switching device second inputpair, and the emitters of transistors Q7 and Q8 coupled together at thesecond terminal of the switching device second input pair.
 7. A mergedvariable gain mixer as recited in claim 6 wherein the four transistorsQ5, Q6, Q7, and Q8 are bipolar transistors.
 8. A merged variable gainmixer as recited in claim 6 wherein the four transistors Q5, Q6, Q7, andQ8 are field-effect transistors.
 9. A merged variable gain mixer havinga double balanced configuration, the merged variable gain mixercomprising: a mixing quad having four transistors Q1, Q2, Q3, and Q4,the bases of transistors Q1 and Q2 coupled to a first terminal, thebases of transistors Q3 and Q4 coupled to a second terminal, theemitters of transistors Q1 and Q3 coupled to a third terminal, theemitters of transistors Q2 and Q4 coupled to a fourth terminal, thecollectors of transistors Q1 and Q4 coupled to a fifth terminal, and thecollectors of transistors Q2 and Q3 coupled to a sixth terminal; and aGilbert current steering quad having four transistors Q5, Q6, Q7, andQ8, the bases of transistors Q5 and Q8 being coupled to a seventhterminal, the bases of transistors Q5 and Q8 further coupled to acontrol voltage input through a resistor R₁, the base of Q6 coupled tothe control voltage input through a resistor R₂, the base of Q7 coupledto the control voltage input through a resistor R₃, the collectors oftransistors Q5, Q6, Q7, and Q8 coupled together at a supply voltageinput, the emitters of transistors Q5 and Q6 coupled together at thethird terminal, and the emitters of transistors Q7 and Q8 coupledtogether at the third terminal.
 10. A merged variable gain mixer asrecited in claim 9 further comprising a first blocking capacitorintended to electrically couple the first terminal to a first localoscillator terminal and a second blocking capacitor intended toelectrically couple the second terminal to a second local oscillatorterminal.
 11. A merged variable gain mixer as recited in claim 10further comprising a third blocking capacitor intended to electricallycouple the seventh terminal to the first local oscillator terminal and afourth blocking capacitor intended to electrically couple the bases oftransistors Q6 and Q7 to the second local oscillator terminal.
 12. Amerged variable gain mixer as recited in claim 9 wherein the transistorsare bipolar transistors.
 13. A merged variable gain mixer as recited inclaim 9 wherein the transistors are field-effect transistors.
 14. Amerged variable gain mixer as recited in claim 9 further comprising abias device capable of providing bias signals at the bases of thetransistors Q1, Q2, Q3, and Q4.
 15. A merged variable gain mixer asrecited in claim 14 wherein the bias device has a bias voltage V_(b),and a resistor R₅ coupling the bias voltage V_(b) to the bases oftransistors Q1 and Q2 and a resistor R₆ coupling the bias voltage V_(b)to the bases of transistors Q3 and Q4.
 16. A merged variable gain mixerhaving a single balanced configuration, the merged variable gain mixercomprising: a mixing stage having two transistors Q1 and Q2; and acurrent steering stage having two transistors Q3 and Q4, wherein themixing stage and the current steering stage are coupled in parallel, theemitters of the transistors Q1, Q2, Q3 and Q4 being coupled togetherforming a current input terminal, the collectors of Q3 and Q4 beingcoupled together at a supply voltage input, and the collectors of thetransistors Q1 and Q2 forming output terminals, wherein the mergedvariable gain mixer arranged for use as an AC coupled merged variablegain mixer, the bases of the transistors Q1 and Q2 being provided a DCbias and further coupled to a local oscillator signal through a pair ofDC blocking capacitors, the bases of the transistors Q3 and Q4 beingprovided a DC bias and a control signal suitable for controlling thegain of the variable gain mixer.
 17. A merged variable gain mixer asrecited in claim 16 wherein the transistors Q1, Q2, Q3 and Q4 arebipolar transistors.
 18. A merged variable gain mixer as recited inclaim 16 wherein the transistors Q1, Q2, Q3, and Q4 are field-effecttransistors.
 19. A merged variable gain mixer having a single balancedconfiguration, the merged variable gain mixer comprising: a mixing stagehaving two transistors Q1 and Q2; and a current steering stage havingtwo transistors Q3 and Q4, wherein the mixing stage and the currentsteering stage are coupled in parallel, the emitters of the transistorsQ1, Q2, Q3 and Q4 being coupled together forming a current inputterminal, the collectors of Q3 and Q4 being coupled together at a supplyvoltage input, and the collectors of the transistors Q1 and Q2 formingoutput termninals, wherein the merged variable gain mixer arranged foruse as a DC coupled merged variable gain mixer switched by a localoscillator signal LO composed of differential parts LO1 and LO2, thelocal oscillator signal LO including DC bias and a gain control signal,the bases of Q1 and Q2 being differentially coupled to LO1 and the basesof the transistors Q3 and Q4 being differentially coupled to LO2.
 20. Amethod for mixing a variably amplifying an input signal I_(f) in orderto generate an output signal O_(f), the method comprising the acts of:receiving an input signal I_(f) having an input frequency and an inputcurrent; receiving a local oscillator signal; receiving a gain controlvoltage V_(gc); absorbing a portion of the input current as a functionof the gain control voltage V_(gc) and the local oscillator signal,thereby generating an attenuated input signal having an attenuated inputcurrent; and generating an output signal O_(f) that is a function of theattenuated input signal, the input frequency, and the local oscillatorsignal, wherein the control voltage V_(gc) can be selected such that theattenuated input current is substantially equivalent to the inputcurrent.
 21. A method for mixing a variably amplifying an input signalI_(f) in order to generate an output signal O_(f), the method comprisingthe acts of: receiving an input signal I_(f) having an input frequencyand an input current; receiving a local oscillator signal; receiving again control voltage V_(gc); absorbing a portion of the input current asa function of the gain control voltage V_(gc) and the local oscillatorsignal, thereby generating an attenuated input signal having anattenuated input current; and generating an output signal O_(f) that isa function of the attenuated input signal, the input frequency, and thelocal oscillator signal, wherein the act of generating an output signalO_(f) includes amplifying the attenuated input current by a constantvalue.
 22. A method as recited in claim 21 wherein the constant value isunity.
 23. A method as recited in claim 21 wherein the constant value isgreater than unity.